This invention relates to a motor vehicle brake drum and particularly to one in which a metal ring is cast in place within the drum structure during production to reduce machining requirements after casting.
Motor vehicle brake drums are typically produced by a casting process in which cast iron is poured into green-sand casting molds. By its very nature, such casting processes have limitations in terms of dimensional tolerances and surface finishes which can be achieved in the as-cast part. It is, therefore, generally necessary to machine the brake drum after casting to generate the inside cylindrical braking surface and to form the mounting surfaces of the center section of the brake drum. As a means of reducing outside contaminants from entering the brake drum in use, brake drums typically have a labrynth groove encircling the open rim of the drum. A sheet metal backing plate mounted to the vehicle axle typically includes an up-standing flange which is positioned within the labrynth groove when the brake drum is mounted to the vehicle. Due to the various dimensional stack-ups and part distortion during use, the labrynth groove of the brake drum must be accurately formed. In some applications, it is possible to form the labrynth groove with sufficient accuracy during brake drum casting. However, in some applications, and especially small passenger car brake drums, the labrynth groove must be formed by machining the casting to provide the requisite degree of accuracy. The requirement of machining the groove results in a significant waste in terms of the material which must be removed from the groove. The machining operation is also demanding in terms of process time and tool life.
The brake drum according to this invention is a composite structure which includes a ring which is insert-cast into the brake drum. The ring insert is preferably made from stamped sheet steel and forms three walls of the labrynth groove and is positioned within the casting mold such that the cast iron material surrounds it thus embedding it within the structure. After casting, the hollow inside surface of the ring defines the labrynth groove. The ring insert can be formed with high precision and located within the molding cavity to eliminate or substantially reduce the requirement of post-casting machining of the labrynth groove. The use of the ring insert is further believed to provide a tighter gap control with respect to the drum backing plate. Moreover, the existence of a steel ring insert within the open end of the drum provides enhanced mechanical properties of the composite drum.
In the prior art there are numerous known designs for composite brake drums including that described in U.S. Pat. No. 4,858,731 issued to the assignee of this invention. In that patent, a formed wire reinforcing structure is embedded within the brake drum structure for the purpose of improving mechanical characteristics, and in particular, the fatigue life of the drum. Various other configurations and processes of forming composite brake drums are known. However, the prior art does not disclose providing the annular ring of this invention for the purposes of forming the surfaces of a labrynth ring for the purpose of providing an accurately formed labrynth seal groove. Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.